John M. Horowitz

Synaptic transmission in nucleus tractus solitarius is depressed by Group II and III but not Group I presynaptic metabotropic glutamate receptors in rats.

Presynaptic metabotropic glutamate receptors (mGluRs) serve as autoreceptors throughout the CNS to inhibit glutamate release and depress glutamatergic transmission. Both presynaptic and postsynaptic mGluRs have been implicated in shaping autonomic signal transmission in the nucleus tractus solitarius (NTS). We sought to test the hypothesis that activation of presynaptic mGluRs depresses neurotransmission between primary autonomic afferent fibres and second‐order NTS neurones. In second‐order NTS neurones, excitatory postsynaptic currents (EPSCs) synaptically evoked by stimulation of primary sensory afferent fibres in the tractus solitarius (ts) and currents postsynaptically evoked by α‐amino‐3‐hydroxy‐4‐isoxazoleproprionic acid (AMPA) were studied in the presence and absence of mGluR agonists and antagonists. Real‐time quantitative RT‐PCR (reverse transcription‐polymerase chain reaction) was used to determine whether the genes for the mGluR subtypes were expressed in the cell bodies of the primary autonomic afferent fibres. Agonist activation of Group II and III but not Group I mGluRs reduced the peak amplitude of synaptically (ts) evoked EPSCs in a concentration‐dependent manner while having no effect on postsynaptically (AMPA) evoked currents recorded in the same neurones. At the highest concentrations, the Group II agonist, (2S,3S,4S)‐CCG/(2S,1′S,2′S)‐2‐carboxycyclopropyl (l‐CCG‐I), decreased the amplitude of the ts‐evoked EPSCs by 39 % with an EC50 of 21 μm, and the Group III agonist, l(+)‐2‐amino‐4‐phosphonobutyric acid (l‐AP4), decreased the evoked EPSCs by 71 % with an EC50 of 1 mm. mRNA for all eight mGluR subtypes was detected in the autonomic afferent fibre cell bodies in the nodose and jugular ganglia. Group II and III antagonists ((2S,3S,4S)‐2‐methyl‐2‐(carboxycyclopropyl)glycine (MCCG) and (RS)‐α‐methylserine‐O‐phosphate (MSOP)), at concentrations that blocked the respective agonist‐induced synaptic depression, attenuated the frequency‐dependent synaptic depression associated with increasing frequencies of ts stimulation by 13–34 % and 13–19 %, respectively (P < 0.05, for each). We conclude that Group II and III mGluRs (synthesized in the cell bodies of the primary autonomic afferent fibres and transported to the central terminals in the NTS) contribute to the depression of autonomic signal transmission by attenuating presynaptic release of glutamate.

A presynaptic mechanism contributes to depression of autonomic signal transmission in NTS

With increasing frequencies of autonomic afferent input to the nucleus tractus solitarii (NTS), postsynaptic responses are depressed. To test the hypothesis that a presynaptic mechanism contributes to this frequency-dependent depression, we used whole cell, voltage-clamp recordings in an NTS slice. First, we determined whether solitary tract stimulation (0.4-24 Hz) resulted in frequency-dependent depression of excitatory postsynaptic currents (EPSCs) in second-order neurons. Second, because decreases in presynaptic glutamate release result in a parallel depression of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-d-aspartic acid (NMDA) receptor-mediated components of EPSCs, we determined whether the magnitude, time course, and recovery from the depression were the same in both EPSC components. Third, to determine whether AMPA receptor desensitization contributed, we examined the depression during cyclothiazide. EPSCs decreased in a frequency-dependent manner by up to 76% in second- and 92% in higher-order neurons. AMPA and NMDA EPSC components were depressed with the same magnitude (by 83% and 83%) and time constant (113 and 103 ms). The time constant for the recovery was also not different (1.2 and 0.8 s). Cyclothiazide did not affect synaptic depression at ≥3 Hz. The data suggest that presynaptic mechanism(s) at the first NTS synapse mediate frequency-dependent synaptic depression.

Functional and anatomical characteristics of the nerve-brown adipose interaction in the rat

SummaryTwo aspects of the coupling of neural information to brown fat thermogenesis were examined—namely, the thermal responses to increasing neural stimulation and the anatomical nature of the brown fat innervation. Upon stimulation of the nerves to the interscapular brown fat pad, there ensued a biphasic response. This response was manifested by an initial, but transient temperature decrease, followed by a rise in brown fat temperature. The magnitude of both components of this response increased with increasing stimulus strength, thereby demonstrating the ability of the tissue to respond in a graded manner—a feature which may underlie the controlled thermogenic response of brown fat observed in the cold-exposed intact animal.No anatomically unique fiber types appeared to be specifically associated with innervation to the brown adipocytes or to the vessels within the fat pad. On the other hand, the nerves entering the interscapular fat pad were morphologically dissimilar, a finding consonent with their functional dissimilarity (i.e., innervation of adipocytes, innervation of blood vessels in the fat pad, and innervation of areas in the overlying skin).

Diurnal modulation of long-term potentiation in the hamster hippocampal slice.

Long-term potentiation (LTP) was examined in hippocampal slices from Syrian hamsters entrained to a LD 14:10 cycle. Population spike (PS) amplitudes from CA1 pyramidal cells were measured before (control) and after tetanizing the Schaffer/collateral commissural pathway. Slices from animals sacrificed during the day, between zeitgeber time (ZT) 0430 and 0530, were incubated, and then tetanized between ZT 1340 and 1930, where ZT=0 denotes lights on. Slices from animals sacrificed during the night, between ZT 1830 and 1930, were incubated, and tetanized between ZT 0030 and 0410. LTP, a sustained increase in PS amplitude following tetanus, was evoked in both groups. PS amplitude increased by 102.7+/-20.3% in animals sacrificed during the day and by 48.0+/-7.5% in animals sacrificed during the night (p<0.05). Thus hamster slices prepared during the day show more robust LTP (a doubling of PS amplitude), a difference persisting in slices incubated for several hours.

Non-NMDA and NMDA receptors in the synaptic pathway between area postrema and nucleus tractus solitarius

Area postrema (AP) modulates cardiovascular function through excitatory projections to neurons in nucleus tractus solitarius (NTS), which also process primary sensory (including cardiovascular-related) input via the solitary tract (TS). The neurotransmitter(s) and their receptors in the AP-NTS pathway have not been fully characterized. We used whole cell recordings in voltage- and current-clamp modes in the rat brain stem slice to examine the role of ionotropic glutamatergic receptors and alpha2-adrenergic receptors in the pathway from AP to NTS neurons receiving visceral afferent information via the TS. In neurons voltage clamped at potentials from -100 to +80 mV, AP stimulation (0. 2 Hz) evoked excitatory postsynaptic currents having a fast component blocked by the non-N-methyl-D-aspartate (NMDA) receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxobenzoquinoxaline-7-sulfonamide (NBQX; 3 microM, n = 7) and a slow component blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 50 microM, n = 8). Although NBQX (3 microM, n = 14) abolished AP-evoked action potentials, APV (50 microM, n = 9 or 500 microM, n = 6) or yohimbine, (200 nM, n = 5 or 2 microM, n = 10) did not. Thus, although AP stimulation activates both non-NMDA and NMDA receptors on NTS neurons receiving TS input, only non-NMDA receptors are required for synaptic transmission.Area postrema (AP) modulates cardiovascular function through excitatory projections to neurons in nucleus tractus solitarius (NTS), which also process primary sensory (including cardiovascular-related) input via the solitary tract (TS). The neurotransmitter(s) and their receptors in the AP-NTS pathway have not been fully characterized. We used whole cell recordings in voltage- and current-clamp modes in the rat brain stem slice to examine the role of ionotropic glutamatergic receptors and α2-adrenergic receptors in the pathway from AP to NTS neurons receiving visceral afferent information via the TS. In neurons voltage clamped at potentials from -100 to +80 mV, AP stimulation (0.2 Hz) evoked excitatory postsynaptic currents having a fast component blocked by the non- N-methyl-d-aspartate (NMDA) receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzoquinoxaline-7-sulfonamide (NBQX; 3 μM, n = 7) and a slow component blocked by the NMDA receptor antagonistdl-2-amino-5-phosphonovaleric acid (APV; 50 μM, n = 8). Although NBQX (3 μM, n = 14) abolished AP-evoked action potentials, APV (50 μM, n = 9 or 500 μM, n = 6) or yohimbine, (200 nM, n = 5 or 2 μM, n = 10) did not. Thus, although AP stimulation activates both non-NMDA and NMDA receptors on NTS neurons receiving TS input, only non-NMDA receptors are required for synaptic transmission.

Thermal effects on long-term potentiation in the hamster hippocampus.

Extracellular CA1 pyramidal cell activity was measured at different temperatures in hippocampal slices from the Syrian hamster (Mesocricetus auratus), a hibernator. Control records taken before and after tetanic stimulation of Schaffer collateral/commissural pathways were compared to determine if long-term potentiation (LTP) was established. LTP (an enhancement of the population spike amplitude or population synaptic response following tetanus) was elicited in slices at temperatures above 22 degrees C, but not in slices at temperatures of 20 degrees C. When LTP was established at temperatures above 24 degrees C, however, lowering the temperature to 20 degrees C did not abolish the LTP. Furthermore, when a tetanus was delivered at 20 degrees C and the bath temperature was then raised above 22 degrees C, LTP was established. These results for step changes in temperature suggest that the sequence of cellular mechanisms leading to LTP is activated, but then arrested in slices maintained at a constant temperature of 20 degrees C. Assuming this type of activity in the slice parallels in vivo hippocampal activity, it follows that the ability to elicit LTP in CA1 hippocampal pyramidal cells is lost when the core temperature of an animal entering hibernation falls to 20 degrees C.

Enhanced adrenergic excitation of serotonergic dorsal raphe neurons in genetically obese rats

Serotonergic neurons in the dorsal raphe nucleus (DRN) project to the ventromedial hypothalamus (VMH), where serotonin (5-HT) release modulates feeding. 5-HT release in the VMH is altered in genetically obese vs. lean Zucker rats. Serotonergic DRN neurons are subject to adrenergic and serotonergic neuromodulation. To determine if the difference in 5-HT release between lean and obese rats might be due to differences in these neuromodulatory pathways, we examined the effects of phenylephrine (PE) and 5-HT on serotonergic DRN neurons using current-clamp recording. Cells from lean and obese animals responded similarly to 5-HT, but cells from obese rats exhibited both a larger depolarization and increased firing rate in response to PE than did cells from lean rats. This indicates that DRN neurons of obese rats have an enhanced adrenergic drive.

Temperature effects on evoked potentials of hippocampal slices from euthermic chipmunks, hamsters and rats.

1. Neural activity was recorded in hippocampal slices from euthermic chipmunks, hamsters and rats. 2. While recording the evoked potentials, the temperature of the Ringers solution bathing the slice was varied by controlling the temperature of an outer chamber jacketing the recording chamber. 3. The temperature just below that at which a population spike could be evoked, Tt, was 10.4 +/- 0.3 degrees C (mean +/- SEM) for chipmunk slices, 14.1 +/- 0.4 degrees C for rat slices and 14.8 +/- 0.4 degrees C for hamster slices. Tt was significantly lower in the chipmunk slices (P<0.01) than in the rat and hamster slices. 4. Data were interpreted as consistent with the hypothesis that chipmunk hippocampal neurons are intrinsically cold resistant.

A Neural Network with a Background Level of Excitation in the Cat Hippocampus

A model based on a limited number of properties of pyramidal cells and interneurons in the CA3 region of the hippocampus has been developed. Elements in this model were selected to correspond to conventional inhibitory and excitatory postsynaptic potentials, neuron thresholds, and conduction delays. Additionally, a signal over pathways making synaptic connections with pyramidal cells was included to provide an ongoing level of excitatory activity. The calculated output of the model was then compared with PST histograms recorded from hippocampal pyramidal cells. The model appeared to account for multiple peaks previously noted in PST histograms recorded over a period of several hundred milliseconds following dorsal fornix stimulation. To test the model, the response of the network was calculated for paired shock stimulation of the fornix and compared with experimental data from the cat. The model was written for CSMP (an IBM program designed to facilitate digital stimulation of continuous processes) and th...

Temperature effects on evoked potentials of hippocampal slices from noncold-acclimated, cold-acclimated and hibernating hamsters

1. 1.|Neural activity was recorded in hippocampal slices from noncold-acclimated, cold-acclimated and hibernating hamsters. 2. 2.|Action potentials from a population of hippocampal pyramidal neurons were evoked by stimulating an afferent fiber tract, the Schaffer collaterals. The temperature of the artificial cerebrospinal fluid bathing the slice was varied by controlling the temperature of a water chamber jacketing the recording chamber. 3. 3.|The temperature just below that at which a population spike could be evoked, Tt, was 15.8 ± 0.9°C (mean ± SEM) for noncold-acclimated hamsters, 13.9 ± 0.3°C for cold-acclimated hamsters and 12.3 ± 0.3°C for hibernating hamsters. 4. 4.|These thresholds for evoked activity were significantly different in noncold-acclimated, cold-acclimated and hibernating hamsters, and may reflect acclimation of hippocampal neurons to cold.